In general, liquid crystal may be classified into rod-type liquid crystal and discotic liquid crystal having a coin shape according to the shape. Of three dimensional refractive indexes nx, ny, and nz of material, the material having at least two different refractive indexes is called a birefringence material, and a direction that is linearly polarized at an incident direction and where no retardation of light occurs is defined by an optic axis. In the rod-type liquid crystal, a long axis direction of a molecule is an optic axis, and in the discotic liquid crystal, a short axid direction of a molecule is an optic axis.
Among them, the alignment state of the rod-type liquid crystal may be largely classified into the following five categories. First, a planar alignment refers to alignment in which an optic axis is parallel in respects to a plane of a film, second, the homeotropic alignment refers to the case of when the optic axis is vertical to a plane of a film, that is, the alignment that is parallel to a normal of a film, and third, a tilted alignment refers to alignment in which an optic axis is tilted at a predetermined angle in the range of 0° to 90° in respects to a plane of a film.
In addition, fourth, a splay alignment refers to alignment in which an optic axis is continuously changed at a tilt angle in the range of 0° to 90°, or a minimum value in the range of 0° to 90°, and fifth, a cholesteric alignment is similar to the planar alignment in that an optic axis is parallel to a plane of a film. The cholesteric alignment refers to alignment in which an optic axis rotates at a predetermined angle clockwise or anticlockwise when it is observed in a direction that is vertical to a plane as it moves in a thickness direction.
Among them, the homeotropic alignment liquid crystal film that belongs to the second alignment state may be used as an optic film such as a retardation film, a viewing angle compensation film and the like in a TN (Twist Nematic) mode, STN (Super Twist Nematic) mode, IPS (In Plane Switching) mode, VA (Vertical Alignment) mode, and OCB (Optically Compensated Birefringence) mode liquid crystal display (LCD) device by using it alone or combining it with another film, and in general, it is manufactured by coating an alignment agent to form a thin alignment film and coating liquid crystal.
In order to attach the homeotropic alignment liquid crystal film to a polarizing plate for brightness improvement or viewing angle compensation, it is required that a roll-to-roll operation in which it is pressed while passing between rollers that are opposite to each other at a predetermined interval like a polarizing plate manufacturing process is performed. In order to perform this, it is preferable that a plastic substrate that is flexible to pressure and a small impact is used.
Some suggestions to form a homeotropic alignment liquid crystal on a plastic film have been given.
U.S. Pat. No. 6,816,218 B1 discloses that an aluminum film that is deposited on a plastic substrate is used as a homeotropic alignment film. In this case, since aluminum is weakly attached to the surface of the plastic substrate, a portion of aluminum is removed when it is stripped, thus it may be a factor of defect.
EP 1376163 A2 discloses that after a liquid crystal solution having horizontal or cholesteric alignment is coated on a plastic substrate, a homeotropic alignment liquid crystal is realized thereon by using it as an alignment film. However, in this case, there is a problem in that according to the degree of curing of the liquid crystal used as the alignment film, the degree of homeotropic alignment of the liquid crystal layer is determined.
US 20060278851 and JP 2006-126757A disclose that a primary amino silane coupling agent is added to a homeotropic alignment liquid crystal solution to form a film having increased attachment strength. However, there is a disadvantage in that this primary amino silane coupling agent makes the alignment of the liquid crystal poor, thus reducing the degree of transparency.
KR 2005-0121835 discloses that an alignment film for deriving a homeotropic alignment of liquid crystal is not separately used but a polymerizable reactive liquid crystal mixture solution including a predetermined surfactant is coated on a plastic substrate, a surface of which is subjected to the hydrophilic treatment to manufacture a homeotropic alignment liquid crystal film. However, there is a big problem in attachment strength between the liquid crystal and the substrate, and since the alignment of the liquid crystal is basically unstable, many defects are formed.